problem regarding null values

hello experts . . .
i've designed a form (two textboxes and one commandbutton)
i wanna display id and user(fields of the table). and i display id and user on the basis of the id(inputted through textbox1).And for that i've written following code within command button's click event

i manually inputted data to the table (i inserted value to id field only and user field remained blank).

and now whenever i click on command button to retrieve data it produces an Error saying "Invalid use of Null".
Now what i want is when there is nothing in the user field then i wanna display the value "0".